Directly heated meshed cathode for electronic tubes and method of making

ABSTRACT

In a directly heated meshed cathode made of one metal piece in the form of a hollow cylinder, the working surface is constituted by intersecting helical filaments 3 and 4 with holes 5 therebetween, each filament 3 and 4 being formed with a stepped increase in the width from periphery to center of the cathode. 
     A method of making the meshed cathode comprises fabrication of a tool electrode out of a plate by electroerosive cutting of grooves in the end portion of the plate with projections therebetween shaped to match the holes 5 between the filaments 3 and 4, and electroerosive broaching, using this tool electrode, of longitudinal rows of holes 5 in a hollow cylindrical blank rotatably displaced, after each pass of the tool electrode, through an angle equal to twice the angular distance between the center lines of adjacent longitudinal rows of holes 5 in the cathode. The grooves are cut out in the plate so that after each pass of the tool electrode, there are produced in the blank: full holes 5 of one longitudinal row, hole-halves 5&#39; of two rows adjoining thereto, and corresponding sections of the filaments 3 and 4 between these holes 5.

FIELD OF THE INVENTION

The present invention relates to vacuum devices, and more particularlyto directly heated meshed cathodes for electronic tubes and to methodsof making.

DESCRIPTION OF THE PRIOR ART

Directly heated tubular meshed cathodes provide for a large currentcapability, due to their extensive working area, as compared to directlyheated rod cathodes. The existing meshed cathode designs, however,suffer from a number of disadvantages that limit their practicalapplication. The main problems with these cathodes include difficultiesin providing uniform emission over the entire working surface, i.e. highcathode efficiency, a long life and cathode parameter stability, as wellas technologically effective designs.

Known in the art is a directly heated meshed cathode with a cylindricalmeshed structure of the working surface formed by intersecting helicalfilaments /cf. FRG Pat. No. 851832, 1950/. In this cathode, all thefilaments are welded together at intersection points, the ends of thefilaments being welded to current-supplying rings.

The method of making such a cathode resides in winding the wire aroundthe cylindrical surface in two directions, welding the wires together atintersections, and welding the wire ends to the current-supplying rings/cf. USSR Inventor's Certificate No. 24491, of 1929/.

The wire meshed cathodes fail to provide a sufficient mechanicalstrength because of the large number of welds, and besides, uniformheating temperature distribution cannot be obtained. The filament endswelded to the current-supplying rings are colder than the central regionof the filaments due to a considerable heat dissipation. Further, themultiple welds cause discontinuities along each filament thus preventingtemperature equalization over the entire working surface of the cathode.Nonuniform distribution of temperature over the working surface of thecathode results in turn in a nonuniform emission current. Moreover, insuch a cathode, the mutually intersecting wires are differntly spacedfrom the cathode axis (in two layers). Therefore, it sets a limit onminimizing the grid-to-cathode spacing. As a result, the possibilitiesof increasing the tube transconductance are limited for the wirecathodes. The manufacturing process of winding the wire with multiplewelds is both complicated and low-efficient. Owing to the nonuniformtemperature distribution, the low mechanical strength, and thestructural discontinuities induced by welding, such cathodes are notpracticable enough and have a short life.

There is also known a directly heated meshed cathode for electronictubes as disclosed in the USSR Inventor's Certificate No. 260748published in 1968, which serves as a prototype of the present invention.This cathode is made of one metal piece in the form of a hollow cylinderwith current-supplying rings provided at its ends, the working surfacebeing confined between the rings and represented by intersecting helicalfilaments with holes therebetween.

Such a cathode has a higher mechanical strength and manufacturingefficiency than the wire cathode. The efficiency of this cathode is alsosuperior to that of the welded wire cathode. One-piece configuration ofthe cathode (made of a single pipe) enables the grid and cathode of sucha tube to be more closely spaced and a uniform grid-cathode spacing tobe provided throughout the entire working surface of the cathode,resulting in a higher transconductance and a wider frequency band of thetube. The cathode may be formed with a varying size of holes between thefilaments, so that the area of the holes of each annular row is lessthan that of the subsequent annular row going in a direction from theperiphery of the cathode to the centre thereof. In this case, the totalsurface area of the filaments in the central region of the cathode isfound to be smaller than the area near the current-supplying rings, thisdifference resulting in some equalization of the emission currentdensity over the cathode surface.

Despite the aforementioned advantageous features of the known one-piecemeshed cathode, however, it still suffers from the inherent defect ofthe known welded wire cathodes in that each helical filament has ahigher temperature in the centre than near the current-supplying rings.The temperature drop along the filament as directed from the centre ofthe cathode towards the rings in the prior art cathode is 400°-500° C.,and consequently, the active portion of the working suface of anyfilament amounts to as little as one-half of its total length. So thearea of the effective emitting surface of the cathode serving as aprototype is approximately equal to half its working surface area, thusradically limiting the power takeoff capabilities of the cathode. It isparticularly the case for the shorter cathodes with the ratio of theworking surface length to the diameter near unity.

The provision of varying size of the holes in the known one-piece metalcathode allows a certain equalization of the integral temperatureprofile over the cathode surface. But even in this case, a temperaturedrop along each filament still occurs without any noticeable increase inthe cathode efficiency, i.e. the nonuniform distribution of emissionover the cathode surface remains. The temperature gradient over thefilament length also reduces the useful life of the cathode.

This cathode may be built using a known method of manufacturing meshedelectrodes for electronic tubes described in the paper by V. N.Alexandrov and V. F. Ioffe "Novye Konstruktsii setochnykh Blokovgeneratornykh i modulyatornykh lamp, oborudovanie dlya ikh izgotovlenia"published in the journal "Obmen Opytom v elektronnoi promyshlennosti",Moscow, issue 7 (17), 1968. According to this method, a tool-electrodeis first fabricated from a plate with the length of its end portioncorresponding to that of the cathode working surface, byelectroerosively cutting out grooves in the ends of the plate using awire electrode, with projections formed therebetween of a shapecorresponding to that of the interfilament holes; this tool electrode isthen employed for electroerosive broaching of longitudinal rows of holesin the hollow cylindrical blank.

The projections formed in cutting the grooves in the plate arelozenge-shaped in cross section and arranged in a single row along theworking surface of the tool-electrode, the width of the plate endmachined for making the tool electrode being chosen equal to thediagonal of the lozenge-shaped interfilament hole perpendicular to thecathode axis, minus two electroerosion gaps. As the hollow cylindricalblank is broached by such a tool electrode, one longitudinal row oflozenge-shaped holes is formed after a single pass of the tool. Theworkpiece is then turned about its axis through an angle equal to theangular distance between the centre lines of adjacent longitudinal rowsof holes in the cathode, and shifted along the axis by a length equal tohalf the other diagonal of the lozenge hole, extending in parallelrelation to the generatrix of the cylinder. After broaching a secondlongitudinal row of holes, the workpiece is rotated through the sameangle and shifted along the axis by the same distance in the reversedirection. In this manner, all the longitudinal rows of holes arebroached, sequentially rotating the work-piece and displacing it eachtime along the axis with respect to the tool electrode. The dimensionsof the cathode holes are here directly determined by the dimensions ofthe projections on the working end of the tool electrode, while thedimensions of the filaments are controlled by the angular displacementof the workpiece around the axis and by its axial displacement withrespect to the tool electrode.

The aforementioned method of manufacture suffers from a number of faultsfurther aggravating the structural disadvantages of the cathode. Amongthe primary defects is a very time-consuming process of fabricating thetool electrode of the desired shape, as well as the inherentlycomplicated mechanism of the equipment employed for broaching the holesin the cylindrical blank due to the necessity of providing highprecision both of angular and axial displacement of the blank. Since inthe known method, the dimensions of the filaments are determined byprecision of angular and axial displacements of the blank, each of themcontributing its individual error, the manufacture of filaments with thedesired accuracy and reproducibility, using this method, presentscertain difficulties. The filaments produced have a large spread inwidth. In operation, additional temperature gradients occur in thecathode manufactured in this manner due to inaccuracies involved infabricating the filaments, thus resulting in a lower efficiency and ashorter life of the cathode.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to improve emissioncharacteristics and to provide a greater case of manufacture of adirectly heated meshed cathode for electronic tubes.

The principal object of the present invention is to provide a directlyheated meshed cathode for electronic tubes that, given a particularsize, should have a larger area of effective emitting surface due to amore uniform temperature profile of the filaments, and to design amethod of making this cathode such as to permit a high-precisionformation of filaments using the simplest technology possible.

With this object in view, there are proposed two structural embodimentsof the meshed cathode, within the scope of the invention, based on theprinciple of a stepwise increase in the surface area of each filamentfrom the periphery of the cathode to the centre thereof.

In accordance with the first embodiment, in a directly heated meshedcathode for electronic tubes made of one metal piece and shaped as ahollow cylinder with current-supplying rings provided at the endsthereof and a working surface confined between the rings and formed byintersecting helical filaments spaced by holes, according to theinvention, each filament features a stepwise width increase from theperiphery to the centre of the cathode.

In accordance with the second embodiment, in a directly heated meshedcathode for electronic tubes made of one metal piece and shaped as ahollow cylinder with current-supplying rings provided at the endsthereof and a working surface confined between the rings and formed byintersecting helical filaments spaced by holes, according to theinvention, in the central region of the working surface of the cathode,between the adjacent intersections of the filaments, there are providedbridges to form at least one equipotential ring parallel to thecurrent-supplying rings.

These bridges should preferably form a number of parallel equipotentialrings, the width of each ring being in excess of the width of eachsucceeding ring, looking from the centre to the periphery of thecathode.

An increase in the effective emitting surface of the first embodiment ofthe cathode structure is due to a higher current density in thosefilament sections disposed closer to the current-supplying rings, thusensuring a more uniform heating of each filament throughout the entirelength thereof. Further, heat dissipation at the filament ends is causedto be reduced as a result of the smaller width of the filaments adjacentthe current-supplying rings.

The larger effective emitting surface in the second embodiment of thecathode structure is due to addition of non-current-carrying bridgesforming equipotential rings to the hot filament sections, causing partof the heat in these sections to be transferred to the bridges. Thisenables the temperature to be equalized along the filaments. Theequalization of temperature in the filaments can be kept within closetolerances by adjusting the bridge width so that the widest bridges beconnected to the hottest portions of the filament.

Each of the two embodiments of the meshed cathode structure is of equalvalue from the viewpoint of attaining the end. According to specificconditions encountered in designing the cathode for a particularelectronic tube, the designer may select either of the proposedembodiments or a combination thereof, i.e. provide a cathode both with astepped increase in the filament width and with equipotential rings.

It will be further noted that both of the proposed embodiments assumeall the cathode filaments to be of equal length. With one-piece metalcathodes, the cross-section of any filament is near rectangular. Thethickness of each filament is uniform along the entire length and theworking surface of all the filaments throughout the cathode is equallyspaced from its axis.

Also with the aforementioned object in view, in a method of making adirectly heated meshed cathode for electronic tubes, includingfabrication of a tool electrode out of a plate by electroerosive cuttingof grooves in the end portion of the plate by a wire electrode, withprojections formed therebetween of a shape corresponding to that of theinterfilament holes, the length of the end portion of said plate beingmachined corresponding to the length of the working surface of thecathode, followed by electroerosive broaching, using this toolelectrode, of longitudinal rows of holes in a hollow cylindrical blankrotatably displaced about its axis after each pass of the toolelectrode, according to the invention, the width of the plate end beingmachined is equal to twice the distance between the centre lines ofadjacent longitudinal rows of holes in the cathode, and the grooves arecut out in the plates so that after each pass of the tool electrode,there are formed in the hollow cylindrical blank: full holes of onelongitudinal row, half-holes of two longitudinal rows adjoining theretoon either side, and two lengths of each filament formed each byintersection between this filament and two other adjacent filaments,each pass of the tool electrode being followed by rotating the blankthrough an angle equal to twice the angular distance between the centrelines of adjacent longitudinal rows of holes in the cathode.

The proposed method of making a meshed cathode provides a simple meansfor high-precision fabrication of filaments, since the formation of theelements of the cathode working surface, when using the proposed method,is controlled by the tool electrode, i.e. the dimensions of thefilaments are determined by the width of the grooves in the toolelectrode and are essentially independent of the accuracy of adjustingthe angular displacement of the workpiece.

In addition, the proposed method provides an increase in productivity toat least twice the output provided by the known manufacturing technique,since the number of tool electrode passes, as the working surface isformed by this method, is equal to half the number of longitudinal rowsof holes in the cathode.

The invention is further illustrated by a detailed description of itspreferred embodiments taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a directly heated meshed cathode for electronic tubes,according to the first embodiment of the invention;

FIGS. 2a,b are temperature profiles of the filaments of the cathodeshown in FIG. 1 and of the known cathode, respectively;

FIG. 3 is a directly heated meshed cathode for electronic tubes,according to the second embodiment of the invention;

FIG. 4 is a tool electrode for fabrication of the cathode of FIG. 1;

FIG. 5 is a view taken along the arrow A in FIG. 4;

FIG. 6 is a cylindrical blank for the cathode after the first pass ofthe tool electrode of FIGS. 4, 5;

FIG. 7 is the same blank after the second pass of the tool electrode ofFIGS. 4, 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The directly heated meshed cathode for electronic tubes, according tothe first embodiment of the invention, is formed by a hollow cylindermade of one piece of metal such as tungsten. At both ends of thecylinder there are provided current-supplying rings 1 (FIG. 1) and 2that confine the working surface of the cathode in the form of a meshedstructure of a length L along the generatrix of the cylinder.

The working surface of the cathode is formed by mutually intersectinghelical filaments comprising a set of parallel filaments 3 directedalong the right helical line and a set of parallel filaments 4 crossingthe same and directed along the left helical line. The filaments 3 and 4are all identical in length and shape, only differing in the directionof the helical lines.

Formed between the filaments 3 and 4 are holes 5 that are lozenge-shapedin the embodiment described.

The meshed structure in the form of the filaments 3 and 4 constitutingthe working surface of the cathode is symmetrical about the centre ofthe cathode shown as a symbolic plane a--a perpendicular to the axis0--O of the cathode of FIG. 1.

According to the invention, each filament 3 and 4 is formed with astepped increase in width looking from the periphery to the centre ofthe cathode. Since all the filaments 3 and 4 are identical with respectto the central plane a--a of the cathode, one half of the filaments 3disposed, say, above the central plane a--a will be consideredhereinafter as exemplifying all the halves of all the filaments 3 and 4;both the top halves adjacent the current-supplying ring 1 and the bottomhalves adjacent the current-supplying ring 2.

The width of the filament 3 is increased stepwise in the direction fromthe top edge of the cathode, i.e. from the current-supplying ring 1, tothe centre a--a.

The portion of the filament 3 closest to the current-supplying ring andformed by the segment 3-1 between the adjacent filaments 4 intersectingthis filament 3 has the lowest width b₁. The next portion of thefilament 3 formed, for example, by the segments 3-2 and 3--3 between twoother adjacent filaments 4 intersecting the filament 3 has a width of b₂which is larger than b₁ but smaller than the width b₃ of the portion ofthe filament 3 lying farther down to the centre of the cathode andcomposed of a sequence of segments 3-4 and 3-5; the width b₃ of thesegments 3-4 and 3-5, in turn, is smaller than the width b₄ of theportion of the filament 3 formed by the segments 3-6 and 3-7. Theportion of the filament 3 formed by the segment 3-8 closest to thecentral plane a--a of the cathode has the largest width b₅, i.e.

    b.sub.5 ≦b.sub.4 ≦b.sub.3 ≦b.sub.2 ≦b.sub.1 =min

Another pattern of increasing the width of the filaments 3 and 4 fromperiphery to centre is also possible. As known from experience, thewidth of the filaments need not always be changed along the entire widththereof. At times, it is sufficient that only those portions adjacentthe current-supplying rings be made of a smaller width than theremaining part of the filament having a constant width. In some cases,however, it may prove inadequate, necessitating the filament to beformed with several portions of different widths, beginning from thecurrent-supplying rings 1 and 2. In this case, the manufacturingefficiency of the structure is ensured by selecting the length of eachfilament portion of uniform width equal to two consecutive segmentsbounded by other filaments intersecting this particular filament, withthe exception of the portions immediately adjoining to thecurrent-supplying rings whose length should be preferably limited by onesuch segment.

For specific cathodes according to the required parameters, the width ofthe filaments 3 and 4 at each portion is calculated by known proceduresconsidering the material properties, the cathode geometry, the lengthand number of filaments, the operational modes of the cathodes, etc.This is generally a computer-aided design. Since it is essentiallyimpossible to give an unambiguous estimate of the interrelation betweena large number of factors controlling the temperature and emissionprofile of the cathode surface, the values computed are subject torefinement. Therefore, the optimum dimensions of the cathode elements,in particular, the widths of the filament portions are finally fittedexperimentally. This is not a very time-consuming job for those skilledin the art, and it is fully justified, since several experiments resultin a cathode of an essentially perfect temperature distribution over thelength of any filament.

FIG. 2a shows a temperature profile throughout the entire length of theworking surface for a cathode made of thoriated tungsten, according tothe embodiment of the invention described. The measurement results inFIG. 2a were obtained by heating the cathode to 2000° K., passing anelectric current therethrough, and measuring the temperature atdifferent points of the filament 3 (FIG. 1) and 4. The curve of FIG. 2arepresents average values for any one filament.

The plot of FIG. 2b characterizing the same function for the knownmeshed cathode as disclosed in USSR Inventor's Certificate No. 260748 isgiven for comparison. As is apparent from FIGS. 2a and 2b, thermaldistribution along the filament is more uniform for the cathode of theinvention than for the known cathode. In the proposed construction ofthe cathode, the effective emission surface area amounts to more than80% of the working surface of the cathode, whereas in the known cathode,this area will be less than 50% of the working surface.

FIG. 3 shows another embodiment of the directly heated meshed cathodeaccording to the invention. This cathode has much in common with thatshown in FIG. 1, i.e. it is likewise constituted by a one-piece metalcylinder, the working surface of the cathode is formed by helicalfilaments 6 directed along the righ-handed helical line and intersectedby helical filaments 7 directed along the left-handed helical line. Thefilaments 6 and 7 are confined between current-supplying rings 8 and 9provided at the cylinder ends. Similarly to the cathode of FIG. 1, theworking surface of the cathode of this embodiment is symmetrical aboutthe centre of the cathode, i.e. about the plane a--a passing through themidpoint of the cylinder generatrix perpendicular to its axis O--O, andthe filaments 6 and 7 have all identical dimensions. The cathode of FIG.3 differs from that of FIG. 1 described above in that each filament 6and 7 has a uniform width over the entire length, and in the centralregion of the working surface between the nearest intersections of thefilaments 6 and 7, in holes 10 therebetween, there are provided bridgesconstituting equipotential rings, of which one ring 11 is disposed inthe centre of the cathode, while the others are arranged in pairs 12 and13, 14 and 15, 16 and 17 symmetrically about the centre a--a of thecathode. The equipotential rings 11, 12, 13, 14, 15, 16, and 17 are allparallel to the current-supplying rings 8 and 9.

The width of the equipotential rings is dependent on the distance fromthe centre a--a of the cathode. The ring 11 disposed in the centre ofthe cathode has the maximum width d₁. The width d₂ of the succeedingrings 12 and 13 is less than the width d₁ of the ring 11; the width d₃of the rings 14 and 15 is less than the width d₂ of the rings 12 and 13,respectively; and the width d₄ of the rings 16 and 17 which are farthestremoved from the centre of the cathode is below the width d₃ of theadjacent rings 14 and 13, i.e.

    d.sub.4 ≦d.sub.3 ≦d.sub.2 ≦d.sub.1 =max

In some cases, e.g. for short cathodes, a single central ring 11 may besufficient.

The number and width of the equipotential rings is also calculated byknown methods using a computer, with the subsequent experimentaloptimization of the values computed.

The equipotential rings provide uniform temperature over the workingsurface of the cathode. These rings carrying no current take up part ofthe heat from all the filaments intersecting the rings, thus minimizingthe temperature at filament/ring intersection points and consequentlyequalizing the temperature throughout the entire length of thefilaments.

The temperature profile of each cathode filament of FIG. 3 is similar tothat of FIG. 2a.

A method of manufacturing a meshed cathode as applied to the structuralembodiment of FIG. 1 is now described.

Prior to cathode manufacture, a tool electrode shown in FIGS. 4 and 5 isfabricated. The tool electrode is made of a copper plate 18. The lengthL' of the end face of the plate 18 is selected depending on the length Lof the working surface of the cathode: L'=L-2x, where "x" is the widthof the electroerosion gap. The width "H" of the end face of the plate 18is chosen to be twice the distance "1" between the centre lines ofadjacent longitudinal rows of the cathode holes 5 (FIG. 1).

Mutually intersecting grooves 19 and 20 are cut out by a wire electrodein the end portion of the plate 18 (FIGS. 4 and 5) using electroerosionmethod. The width and arrangement of each of the grooves 19 and 20correspond to those of that one section of the filament 3 (FIG. 1) or 4of the cathode formed by two consecutive segments bounded by otherfilaments intersecting this particular filament. The choice of the widthY_(n) of each groove 19 (FIG. 4) and 20 is based on the condition Y_(n)=b_(n) +2x, where b_(n) is the width of the corresponding region of thefilament 3 (FIG. 1) or 4, and "x" is the width of the electroerosiongap. The grooves 19 (FIG. 4) and 20 of different widths are cut outeither by the wires of a varying diameter or by the wire of the samediameter with varying manufacturing techniques, or else displacing itwithin the groove following a predetermined program.

The depth "K" of the grooves 19 and 20 is determined from the conditionK≧z N/2, with "Z" the wall thickness of the blank for the cathode, and"N" the number of longitudinal rows of holes 5 (FIG. 1) in the meshedstructure of the cathode. The degree of wearout of the tool electrode isalso taken account of in selecting the depth "K" of the grooves 19 (FIG.4) and 20.

After all the grooves 19 and 20 have been cut out in the end portion ofthe plate 18, three longitudinal rows of projections 21, 22, 23 arefound to exist between these grooves. Now the projections 21 of themiddle row correspond, in cross section, to the full holes 5 (FIG. 1) ofone longitudinal row of the cathode holes, while the projections 22(FIG. 4) and 23 of the end rows correspond to halves of holes 5'(FIG. 1) of the row adjacent to the first longitudinal row of cathodeholes (allowing for electroerosion gaps).

As clearly seen from FIG. 4, when the grooves 19 and 20 are cut,triangular lugs 24 and 25, shown as dashed lines, are formed near theshort sides of the rectangular end face of the plate 18. These lugs areto be removed, since they are liable to be displaced on account of theirinsufficient rigidity, thus resulting in a lower accuracy of fabricationof the most critical regions of the filaments adjacent thecurrent-supplying rings.

If the tool electrode is manufactured from a more rigid material such assteel or titanium, the lugs 24 and 25 may be left. In this latter case,the meshed structure of the cathode will have an appearance differentfrom that shown in FIG. 1.

Further, a hollow cylindrical blank 26 (FIG. 6) is taken; its length anddiameter corresponding to the required length and diameter of thecathode, respectively, and the thickness of the walls being equal to thespecified thickness "z" of the cathode filaments. Longitudinal rows ofholes are sequentially broached in this blank 26 by means of theprefabricated tool electrode shown in FIGS. 4 and 5 using electroerosiontechnique. In the course of a single pass of the tool electrode, thefull holes 5 (FIG. 6) of one longitudinal row are caused to be formed inthe blank 26 (FIG. 6) by the projections 21 (FIG. 4), while thehole-halves 5' (FIG. 6) of two rows adjoining to that row on both sidesare formed therein by the projections 22 (FIG. 4) and 23.

The blank 26 is then rotated around its axis through an angle of β (FIG.7) equal to twice the angular distance α (FIG. 1) between the centrelines of the holes 5 of adjacent longitudinal rows. Again this isfollowed by forming, within a single pass of the tool electrode, thefull holes 5 of one longitudinal row and the hole-halves 5' of two rowsadjoining thereto on both sides, as shown in FIG. 7. Consequently, thehole halves 5' of the row disposed between the rows of the full holes 5obtained within the first and second consecutive passes of the toolelectrode are made complete, so that three rows of full holes are foundafter the second pass of the tool electrode.

The process described is repeated in the same sequence until the wholestructure of the cathode working surface of FIG. 1 is ultimately formed.

When the longitudinal rows of holes 5 are broached in the blank 26, asshown in FIGS. 6 and 7, the dimensions of the sections of the filaments3 and 4 are essentially not dependent on the accuracy of rotation of theblank 26; rather, they are directly determined by the dimensions of thegrooves 19 and 20 (FIGS. 4 and 5) of the tool electrode that can bemaintained within close tolerances considering the present state of theart of electroerosion technology using a nonshaped wire electrode.

The meshed cathode shown in FIG. 3 is manufactured in a similar fashion,except that in producing the tool electrode, all the intersectinggrooves are cut to the same width, and additional grooves of a varyingwidth are cut out in parallel relation to the shorter sides of the endface of the plate 18 (FIG. 4) to form bridges constituting theequipotential rings 11 to 17 (FIG. 3).

The machining regime in fabrication of the tool electrode and formationof the cathode structure is selected following an accepted procedure asapplied to specific needs, and also accounting for capabilities of theequipment employed.

INDUSTRIAL APPLICABILITY

The invention can be extensively used in electro-vacuum industry formanufacture of generator and modulator tubes. The embodiments of themeshed cathodes described above enable the efficiency of the directlyheated meshed cathodes to be increased by a factor of 1.3 to 1.5. Inorder that the same emission current values be achieved that occur inthe existing cathode designs, a smaller specific heating power will berequired, and consequently, a lower filament temperature. As a result,the useful life of the proposed cathode is 3 to 5 times as long as thatof the known designs. The implementation of the invention opens the wayto providing very reliable, low-cost, and long-lived electronic tubes.

We claim:
 1. A directly heated meshed cathode for electronic tubes, saidcathode being made of one piece of metal in the form of a hollowcylinder with integrally formed current-supplying rings provided atrespective ends thereof and a perforated working surface defined betweenthe rings by intersecting incandescent filaments spaced by rhomboidalholes, each of the filaments having the form of a spiral conductorinterconnecting the respective current-supplying rings, and each of theincandescent filaments constituting a series of successive lengths, eachof which being restricted by intersections of said incandescent filamentand two other incandescent filaments, each of the spiral incandescentfilaments having a discretely varying cross-sectional area along saidsuccessive lengths for which purpose each length of the incandescentfilaments has at least one step-wise increase in width in a directionfrom each of the current-supplying rings toward the middle of saidincandescent filament.
 2. A method cathode as claimed in claim 1,wherein each two adjacent lengths of each of the incandescent filaments,except for the lengths immediately adjoining the current-supplyingrings, form portions having a constant width which is greater than thewidth of said lengths immediately adjoining the current-supplying rings.3. A meshed cathode as claimed in claim 2, wherein the width of each ofsaid portions of the incandescent filament is greater than the width ofthe preceding portion of said incandescent filament in the directionfrom the current-supplying rings toward the middle of the filament.
 4. Adirectly heated meshed cathode for electronic tubes, said cathode beingmade of one piece of metal in the form of a hollow cylinder withcurrent-supplying rings provided at respective ends thereof and aperforated working surface defined between the rings by intersectingincandescent filaments spaced by rhomboidal holes, each of the filamentshaving the form of a spiral conductor interconnecting the respectivecurrent-supplying rings, and each of the incandescent filamentsconstituting a series of successive lengths, each of which beingrestricted by intersections of said incandescent filament and two otherincandescent filaments, each of the spiral incandescent filaments havinga discretely varying cross-sectional area along said successive lengths,for which purpose the width of each length of the incandescent filamentsis discretely increased near the intersection of the incandescentfilaments by at least one bridge being formed from the same piece ofmetal and being peripherally spaced between respective intersections ofthe spiral incandescent filaments, said bridges forming at least oneequipotential ring which is parallel to the current-supplying rings andequally spaced from said current-supplying rings.
 5. A meshed cathode asclaimed in claim 1 including a plurality of bridges constitutingparallel equipotential rings, the width of each of said bridges beinggreater than the width of a successive bridge in the direction from themiddle of the incandescent filaments toward the current-supplying rings.6. A method of making a directly heated meshed cathode for electronictubes comprising the steps of:making an electrode tool by performingrhomboidally-shaped projections in the working end of a rectangularmetal plate having the length of the working end corresponding to thelength of the working surface of the cathode being manufactured; usingthe electrode tool thus obtained to broach longitudinal rows of holes ina hollow cylindrical blank of the cathode by electric erosion, passingsaid tool parallel to the axis of said blank while also moving said toolradially in the course of the broaching operation relative to thecathode blank; turning said blank periodically about the axis thereoffollowing every pass of the electrode tool; said method furthercomprising the steps of: selecting said rectangular metal plate so thatthe width of its working end is equal to the double distance between themiddle lines of the adjacent longitudinal rows of holes in the cathodebeing manufactured; and said projections, cutting blind intersectinggrooves in the working end of said plate with a width and locationpattern corresponding to the width and location pattern of portions ofthe incandescent filaments of the cathode being manufactured; thereby tobroach open holes in said cathode blank so that following each pass ofsaid electrode tool, full holes of one longitudinal row, a half of theholes of the two longitudinal rows adjoining it on both sides thereofare formed in the cathode blank simultaneously with portions ofincandescent filaments of the cathode, spaced by the formed holes andeach consisting of filament lengths, each of which being restricted bythe intersections of said incandescent filament with two otherfilaments, said cathode blank being turned in the turning step,following each pass of said electrode tool, about its axis through anangle equal to twice the angular distance between the middle lines ofthe longitudinal rows of holes in the cathode being manufactured.
 7. Amethod as recited in claim 6 wherein, during the forming step, saidelectrode tool is formed by electric erosion using a wire electrode.